In recent years, small-size, light-weight, and high energy-density secondary batteries have been in demand as power sources for electronic equipment including audio visual equipment, personal computers and the like, and for communications equipment. On the other hand, in large-size batteries including batteries for electric vehicles, research and development in the area of environmental problems are being conducted and high-capacity, high-output, and high-voltage secondary batteries have been requested. Among these batteries, lithium secondary batteries are greatly expected. Especially in large-size lithium secondary batteries, vibration resistance is required so that no problems of connections such as breakage of lead plates due to vibration will occur when on board a vehicle as a power supply, in addition to the requirement for improvements in large-current load characteristic accompanying increasingly higher output and in longer life.
Generally speaking, lithium secondary batteries which are currently being the mainstream batteries employ in the positive electrode complex oxide of lithium and transition metals such as lithium cobaltate, lithium nickelate, and lithium manganate, and in the negative electrode carbon material which can intercalate and de-intercalate lithium ions, and a non-aqueous electrolyte as the electrolyte. As the positive electrode potential of these lithium secondary batteries is as high as 4 volts or greater, aluminum (Al) which has a high-voltage resistance and a high corrosion resistance is usually used as the collector material of the positive electrode and the structural material of seal plate and the like. Also, in the negative electrode, copper (Cu) which has a superior electrical conductivity is generally used.
Also, to each of the band-shaped positive electrode and negative electrode, a lead plate is generally connected by welding and other method at the central or end portion. An electrode group is made by laminating these electrodes with a separator interposed and spirally winding, and the lead plates are electrically connected to collector terminals by welding and other method as shown in FIG. 6 thus allowing to take out a current through the lead plates.
With large-size batteries, there has been a demand for improvement in the load characteristic in association with the trend toward higher output. In this case, it is necessary to increase the area of the electrodes so that the current density per unit area of the electrodes will not become excessive. However, in practice, there being a certain limit to increasing the electrode areas by increasing the dimension in the direction of the height of the unit cell, namely, in the direction of the width of the electrodes, improvement of the load characteristic at higher outputs is being performed by increasing the length of the electrodes.
In connecting a multiplicity of large cells, cable and other connecting members are sometimes affixed using a collector terminal on which a bolting portion has been formed. In doing this, when aluminum (Al) is used for the collector terminal, strength-wise failure is sometimes caused as the bolt is easy to be broken when a nut is screwed or deformation of the connecting section results due to the compression of the foot section of the bolt.
Also, aluminum (Al) is easy to be oxidized causing an increase in the electrical resistance and an accompanying decrease in the electrical conductivity. It is generally considered difficult to apply plating such as nickel (Ni) plating to prevent oxidation. To address these problems, there is a method to form the section having the bolting portion with stainless steel and the like which has a greater tensile strength than aluminum (Al), and the other section with aluminum (Al), and fasten them with a screw to avoid from becoming loose.
In this case, although the weak point of breakage of the bolting portion due to fastening of the nut is remedied by using a high strength material, it has a disadvantage of causing an increase in the resistance of the terminal section due to resistance between different types of metals and of having insufficient hermeticity.
The present invention addresses the above-described problems and aims at providing a collector terminal for a non-aqueous electrolyte battery provided with a high-reliability bolting terminal with which the bolting portion is not easily broken or its foot portion does not become loose even when the bolt is fastened with an excessive torque thus maintaining the characteristic of aluminum (Al) superior in high-voltage resistance and in high corrosion resistance.
With regard to the electrode structure, as the volume the electrodes can occupy inside a battery case is limited, even though the thickness of the electrodes becomes smaller and the current density per unit area of the electrodes decreases as their length is made longer, the distance to the lead plates becomes greater by the amount the area has been increased and the electrical resistance becomes greater, thus not fully exhibiting the merit of increasing the electrode area.
There exists a method to solve this problem by affixing two or more lead plates in a group onto the same electrode as illustrated in FIG. 7, and taking out the lead plates in parallel in the same direction for connection with the collector terminal. With this structure, though the above problem may be solved, there is a risk of the lead plates being broken due to vibration when used on board a vehicle as a power supply.
The present invention addresses these problems and provides a highly vibration-resistant non-aqueous secondary battery which is free from troubles in the connecting section such as breakage of the lead plates due to vibration, shock, and so on, especially in a large size battery.